1. Field of the Invention
The present invention relates to an electronic imaging apparatus such as a digital still camera, and in particular relates to an imaging apparatus and an imaging method capable of generating a bulb exposure image with relatively bright composition and relatively dark composition, and capable of arbitrarily adjusting exposure time.
2. Description of Related Art
Conventionally, with a single lens reflex type imaging apparatus, observing a subject image has been performed using an optical viewfinder. However, there has been recently known an imaging apparatus that eliminates an optical viewfinder and permits a user to observe a subject image through live view display for displaying an image that has been read out from an image sensor using a liquid crystal monitor or the like. In addition, instead of an optical viewfinder, imaging apparatuses have become available on the market that display an image through live view display using an electronic viewfinder that is separate from a liquid crystal monitor, and are capable of switching between the liquid crystal monitor and the electronic viewfinder.
Conventionally, regardless of whether an optical viewfinder or an electronic viewfinder is used, at the time of a prolonged exposure, such as with a bulb exposure, it was not possible to read out an image signal from the image sensor during exposure, which meant that it was not possible to confirm the state of the subject or exposure conditions during exposure, and an image could only be confirmed only upon completion of shooting. As a result, for the photographer, setting of exposure and exposure time and deciding when to start and stop the exposure need to be determined by the photographer themselves through estimation from subject brightness and the like, and it is not an easy matter to obtain a desired exposure image without insufficient exposure or over exposure.
Thus, an imaging apparatus that reads out a pixel signal from an image sensor at a predetermined time interval and displays an image obtained by simple cumulative addition on a liquid crystal monitor each time an image signal based on this pixel signal is read out of the image sensor is proposed. (See Japanese Patent Laid-Open No. 2005-117395 (hereinafter referred to as “Patent Literature 1”).) According to this imaging apparatus, since the course of exposure is displayed during prolonged exposure such as bulb exposure, a failure of shooting can be reduced. Moreover, there is proposed an imaging apparatus in which an image signal is continuously read out of the image sensor, and a bulb exposure image is generated with relatively bright composition (a composition method in which brightness levels of each pixel of the image data are compared, and the pixel with larger brightness level is selected as the brightness level after the composition). (See Japanese Patent No. 4148586 (hereinafter referred to as “Patent Literature 2”).)
In the bulb exposure, shooting is carried out by exposure for a long time from several seconds to several minutes (referred to as “long exposure shooting”). As characteristics of the image sensor, a dark current component is generated in a photodiode constituting a pixel of the image sensor during the long exposure shooting, and a fixed pattern noise is generated. A generation amount of the dark current varies for each pixel and increases in proportion to exposure time. Moreover, it tends to increase as a temperature of the image sensor is higher, which appears as a defect-state noise or uneven image density on an image.
This fixed pattern noise caused by the dark current is generated depending only on the image sensor temperature during exposure and exposure time, whether it is shooting in an exposure state with a shutter open or shooting in a light-shielded state. Thus, in the digital camera currently available on the market, FPN (Fixed Pattern Noise) cancellation processing is executed in which, in bulb exposure, if a user performs shooting (an image taken at this time is referred to as a “bright image”), a light-shielded image is automatically shot at the same shutter speed in a state in which the shutter is closed after this shooting is finished, and the bright image data and the light-shielded image data are subjected to subtraction processing in an image processing circuit in a rear stage of the image sensor so as to correct the fixed pattern noise.
However, when a user performs bulb exposure using the imaging apparatus described in Patent Literatures 1 and 2 or the digital camera recently available on the market, the user starts shooting after shooting setting such as check of composition, focusing and the like while checking an image displayed through live-view display before the shooting. Moreover, in order that the user can concentrate on shooting including composition setting without feeling a sense of discomfort caused by seeing the live-view display and directly seeing the subject with the naked eye, the image is displayed at a high-speed frame rate (30 frames, 60 frames, or more, per second) from the image sensor for the live view. This causes the temperature of the image sensor to increase.
On the other hand, when the bulb exposure is started, the number of times of the reading-out operation from the image sensor is reduced during an exposure operation, and an operation with suppressed power consumption for a part of circuits in the image sensor is performed and thus, the consumed power, that is, heat generation from the image sensor is considerably suppressed as compared with the operation in the live view. Moreover, during a period when image reading-out is not carried out, processing is not executed in the image processing circuit or display of the live-view image is not updated, either, and thus, heat generation in the camera as a whole is suppressed. Thus, if shooting is performed in a place without a change in an environmental temperature, the temperature is high at start of the bulb exposure and the temperature tends to decrease as the exposure continues.
With the imaging apparatus described in Patent Literatures 1 and 2 and the digital camera with live-view function available on the market, since a dark image is shot after shooting of the bright image, the dark image is shot in a state in which the temperature of the image sensor is low. Since the dark current causing the fixed pattern noise increases as the temperature of the image sensor becomes higher, the bright image has a larger fixed pattern noise than the dark image, and it cannot be corrected even if the FPN cancellation processing is executed.